Chronic wasting disease (CWD)
Chronic wasting disease (CWD) is a fatal neurological disorder that affects deer, elk, and moose, primarily found in North America. It is caused by infectious proteins known as prions, which lead to the progressive degeneration of the brain and nervous system, ultimately resulting in death. The disease was first identified in Colorado in the late 1960s and has since been detected in numerous states across the U.S. and in parts of Canada, as well as in Europe.
Infected animals exhibit a range of symptoms, including severe weight loss, listlessness, increased salivation and urination, and depression. While prion diseases are known to affect multiple species, there are concerns regarding the potential for CWD to transmit to humans, although research suggests a significant species barrier exists. CWD spreads primarily through contact with infected body fluids, especially saliva and blood. As the disease continues to spread, discussions around managing its impact are growing, including potential legislation and preventive measures to protect both wildlife and cultural practices dependent on deer populations. Current research aims to develop effective diagnostic tests and better understand the disease's transmission and pathology.
Subject Terms
Chronic wasting disease (CWD)
ANATOMY OR SYSTEM AFFECTED: Brain, nervous system
DEFINITION: A neurological disease of deer and elk caused by an infectious protein particle called a prion. Prion diseases include chronic wasting disease, bovine spongiform encephalopathy ("mad cow" disease), and Creutzfeldt-Jakob disease.
CAUSES: Prion disease in deer and elk
SYMPTOMS: In animals, progressive weight loss, listlessness, increased salivation and urination, increased water consumption, depression, death; possible transmission to humans through infected game
DURATION: Chronic and progressive
TREATMENTS: None
Causes and Symptoms
Chronic wasting disease (CWD) is an invariably fatal neurological disease that affects deer, elk, and moose in North America. The disease was first described in the late 1960s in captive animals in Colorado. As of March 2024, according to the Centers for Disease Control and Prevention (CDC), CWD-infected animals from wild populations had been found in at least thirty-two states in the United States and in four provinces in Canada. The disease had also been detected in captive populations in some states and provinces and had been reported in Norway and Finland. In the late stages of the disease, infected animals show progressive weight loss, listlessness, increased salivation and urination, increased water consumption, depression, and death.
The brains of dead animals show characteristic vacuoles, or holes, that give the brain the appearance of a sponge. This pathology is characteristic of all the transmissible spongiform encephalopathies (TSEs), including bovine spongiform encephalopathy (BSE), more commonly known as mad cow disease; scrapie, found in sheep; and Creutzfeldt-Jakob disease in humans. Although CWD is similar to these other diseases, it is not identical to them. These diseases appear to be caused by the misfolding of a small protein normally located in the membranes of neurons and other cells. The normal function of this protein is unknown, but when misfolded, it forms prions that accumulate in the brain, resulting in the death of neurons. The misfolded, pathogenic prion appears to be able to direct the misfolding of any normal prion present in a cell, thus replicating its aberrant structure.
Prion diseases can be either inherited or transmitted. Humans can inherit Creutzfeldt-Jakob disease or contract it from contaminated blood or brain tissue. Evidence suggests that variant Creutzfeldt-Jakob disease in humans can result from consuming beef contaminated with the BSE prion. There is concern about whether the CWD prion in deer and elk can also “jump” the species barrier to humans in a manner similar to that of BSE. Research using monkeys suggests that there is a species barrier to humans, making it unlikely that CWD will be transmitted to humans despite contact with contaminated blood or tissues.
CWD is believed to be transmitted from animal to animal through contact with body fluids or feces. Research published in 2006 indicated that the disease is most easily transmitted via saliva and blood, although other modes of transmission have not been ruled out. Although two studies found no strong clinical or epidemiological evidence that the disease can be transmitted to humans via the consumption of contaminated meat, laboratory studies indicate that such transmission is possible. Because of the long incubation period for the disease, however, it is difficult to prove or disprove transmission from deer, elk, or moose to humans.
As occurrences of CWD in both wild and captive populations had continued to spread into more states by early 2024, some, including in states such as Minnesota where deer are crucial to inhabitants' culture and diet, have begun to discuss possible methods for dealing with the threat. While some proposed developing faster, easier field tests, others considered further legislation and regulations to at least help prevent the spread of the disease. In 2023, the bipartisan Animal Disease and Disaster Prevention, Surveillance, and Rapid Response Act was introduced.
Perspective and Prospects
Many states where CWD is prevalent have banned the baiting of deer and are thinning the deer population. Deer and elk in crowded populations, particularly captive herds, appear to be more susceptible to CWD.
Research efforts are focused on developing a sensitive field diagnostic test. Definitive diagnosis is made by necropsy, but tests based on tonsil biopsy and antibodies to detect prions in body fluids are also being developed. Transgenic mouse models of each of the known TSEs are also being created in order to learn more about transmission and pathology.
Bibliography
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